The use of underlift and wheel lift towing apparatus is well known. Such apparatus are disclosed in Brown, U.S. Pat. No. 4,473,334; Hamman, U.S. Pat. No. 4,634,337; Bubik, U.S. Pat. No. 4,637,623; and Bubik, U.S. Pat. No. 4,793,763, and these patents are incorporated herein by reference. Often, such apparatus may be folded into an upright, stored position. (See, for example, Brown, U.S. Pat. No. 4,473,334.) These apparatus usually employ a hydraulic cylinder which, as it is extended, rotates the vehicle lifting boom (also known as a "stinger") about a pivot point into the stored position.
Because it is desirable to maximize the ground clearance of the stinger assembly while in use or when stored, and because the hydraulic cylinder extends down toward the ground as it lifts the stinger, it is important to bring about this folding-storing motion with a minimum of extension of the hydraulic cylinder. This may be accomplished by locating the axis of rotation of the lifting boom close to the connecting point between the hydraulic cylinder and boom. Thus, a small extension by the hydraulic cylinder causes a large movement by the stinger into the upright, stored position. However, as a result of this short moment arm between the hydraulic cylinder and the axis of rotation such a mechanism may suffer from a significant lack of lifting capacity.
In many situations, the vehicle to be towed is not on a level plane with the towing vehicle, e.g., the vehicle may be downhill or in a ditch. In certain cases, the undercarriage of the vehicle to be towed is configured such that in order to lift the vehicle without damage, the stinger must be tilted in order to properly engage the vehicle. While many stinger assemblies are capable of tilting "positively"--from a horizontal position toward a higher position--to engage the vehicle to be towed, many are not capable of "negative" tilt--that is, dropping the stinger below the level of the axis of rotation of the boom in order to properly engage the vehicle. The inability to provide "negative" tilt is a significant drawback in prior equipment.
Furthermore, after the stinger has engaged the vehicle, it is oftentimes desirable to tilt the stinger to reach the optimum lifting position which often requires substantial additional power. However, some prior art apparatus, which employ a single hydraulic cylinder to move the stinger, lack this additional required power.